The process of applying or coating liquids onto substrates or webs is well known. However, the process can be complex depending on the liquid and the substrate used, on the performance objectives of the end product, and on the process itself. Many coating apparatus and coating process variations have been developed to address specific coating needs.
U.S. Pat. No. 2,681,294 discloses a vacuum method for stabilizing the coating bead for direct extrusion and slide types of metered coating systems. Such stabilization enhances the coating capability of these systems. However, these coating systems lack sufficient overall capability to provide the thin wet layers required for some coated products, even at very low liquid viscosities.
U.S. Pat. No. 4,445,458 discloses an extrusion type bead-coating die with a beveled draw-down surface to impose a boundary force on the downstream side of the coating bead and to reduce the amount of vacuum necessary to maintain the bead. Reduction of the vacuum is noted to minimize chatter, streaks, and other coating defects. Coating quality is noted to be further improved by optimizing the obtuse angle of the beveled surface with respect to the slot axis and the position along the slot axis of the bevel toward the moving web (overhang) and away from the moving web (underhang). The optimization results in the high quality desired for coating photosensitive emulsions. However, the thin-layer performance capability desired for some coated products is lacking.
A common problem encountered with extrusion die coaters and with slide coaters has been the occurrence of streaking of the liquid when applied to the substrate. One cause of streaking is dried liquid residue on the die lips near the coating bead. This cause and the resulting streaking problem are especially prevalent for low-viscosity liquids containing a highly-volatile solvent.
European Laid Open Patent Application EP 0 581 962 A1 describes one approach for reducing coating defects. A eutectoid dispersion of a fluorine-containing resin (such as Teflon.TM.) and nickel is plated onto the die faces and lips of the coating apparatus. Upon curing, this surface is reported to provide the hardness and dimensional stability necessary for a coating die while also keeping the water-repelling property of a fluorinated surface. This treatment is noted to prevent wetting of the die surface by the coating liquid and reduces streaking, dripping, and edge waviness in the coating.
U.S. Pat. No. 5,380,365 describes covering or coating a surface of a slide coating die adjacent to and below the coating bead with a low energy material, such as a fluorinated polyethylene. The covering starts 0.05-5.00 mm below the coating lip tip and extends away from the coating bead. The low surface energy covering is separated from the coating lip tip by a bare metal strip. This locates the bead static contact line. The low energy covering is noted to eliminate coating streaks and to facilitate die cleanup.
A number of known compounds are described in such patents as U.S. Pat. No. 3,787,351 (Olson). This patent describes oligomers containing fluoroaliphatic groups and poly(oxyalkylene) solubilizing groups. These oligomers are said to be useful as wetting agents in order to improve the mechanical properties of shaped articles of filled resin composites, e.g., glass-reinforced polyester or epoxy resin composites. Further, U.S. Pat. No. 4,415,615 (Esmay et al.) discloses the use of some of the oligomers of U.S. Pat. No. 3,787,351 as surfactants in the preparation of cellular pressure-sensitive adhesives in order to produce a uniform cell structure.
Non-fluorinated polymerizable surfactants have been described, e.g., in U.S. Pat. No. 4,560,599 (Regen). That patent describes a method for direct stepwise coating of a solid substrate with a polyfunctional polymerizable surfactant. Among the polymerizable surfactants used are methacrylate-functional phosphatidyl cholines and phosphate esters.
Zh. Fiz. Khim., 1982, 56, 2898 (Abstract) describes fluorine-containing acrylates of the formula CH.sub.2 .dbd.CHCO.sub.2 CH.sub.2 (CF.sub.2 CF.sub.2).sub.n R, wherein R is H or F, and n is 1 to 4, as being useful for imparting hydrophobicity and chemical resistance to the surface of polymers by copolymerization.
U.S. Pat. No. 5,468,812 (Muggli et al.) describes polymeric, oligomeric fluorochemical surfactant compositions having at least two pendent fluoroaliphatic groups, at least two organic-solubilizing groups, and a pendent polymerizable olefinic group (prepared from a bifunctional monomer). The compositions reduce the surface energy of acrylate-based pressure-sensitive adhesives. They can be used in preparing acrylate-based pressure-sensitive adhesives that coat well due to the presence of the surfactant and maintain adhesive performance upon aging.
However, a need remains for a streak-reducing material which withstands the abrasion and/or the impact which occur during normal working conditions. Abrasion resistance can be particularly important when the liquid being coated has an abrasive quality and when clean-up involves wiping surfaces with cloths, brushes, and the like. Impact resistance can be particularly important when the coating process can include instances when the web or substrate breaks and strikes the material.
The durable streak-reducing material should be easily, quickly, and cost-effectively applied. The application process should not require that the coating apparatus be taken off-line for a significant duration to apply the material. Preferably, the application process is relatively quick and can be accomplished at or near the coating site, rather than requiring the coating apparatus to be shipped to a facility which has highly specialized capabilities.
In addition, the process of applying the streak-reducing material should not involve a significant risk of detrimentally affecting the coating apparatus. For example, the process should not run the risk of dimensionally distorting the coating apparatus by requiring the coating apparatus to be raised to a temperature above a critical level.
The process of applying the streaking-reducing material also should involve no more than a moderate capital investment, and the process and the material itself should provide for only a moderate cost to actually apply the material to the coating apparatus.